Human hair is made up of a complex protein called alpha-keratin. Protein molecules of the hair are arranged in organized patterns and are held together by weak bonds, such as hydrogen, saline and hydrophobic bonds, and stronger ionic bonds and sulphur bridges. These bonds lend stiffness or rigidity to the hair and enable styling of the hair with waves and curls as these bonds are broken and then re-established in different orientations through the styling process. As is known, styling of hair may be accomplished by breaking the bonds by adding energy to the hair, such as by heat with a curling iron or blow dryer, or by getting the hair wet or damp. When the hair is wet or damp and the hydrogen bonds are broken, the hair becomes elastic and can be stretched and given a particular form since the position of the keratin chains has been altered. As the hair dries, the bonds reform in different places, maintaining the hair in its new shape. Blow-drying or setting assists in controlling the styling process so that, once dry, the hair will retain the form it has been given.
Hair is hydroscopic and permeable so it will absorb water from the environment. Under normal conditions, water accounts for about 12% to 15% of the composition of hair. Normal hair can absorb more than 30% of its own weight in water. If the hair is damaged, this percentage can approach 45%, however damaged hair has less ability to retain water within the hair fibers which gives hair its healthy appearance. As more water is absorbed within the hair fiber due to humidity or prior damage, the hydrogen bonds may loosen so that the hair has a decreased ability to maintain its set.
During styling, if the hair is too wet, it will not hold its shape and water must be removed before styling will be effective. Conversely, when the hair is too dry, the hydrogen bonds will have already been formed and poor styling will result since the keratin chains cannot be repositioned and set. It has previously been determined that optimum styling results may be achieved when the moisture of hair is in the range of approximately 30-40% by weight. It is thus desirable to be able to tell when to begin styling (i.e., when moisture in the hair is in a range of 30-40%) to obtain the optimum styling results.
Likewise, it is also important to know when to stop styling hair which has been wetted to break the hydrogen bonds. If the hair is too dry, it will not be flexible and potential damage of the hair may result when styling is continued. It has been determined that the process of drying hair exhibits two stages which are relevant to styling. In a primary drying stage, water is evaporated from the outside of the hair fibers and no styling benefit is achieved. In a secondary drying stage, water from inside the hair fibers is diffused to the environment. It is during this transition to the secondary drying stage when optimum styling of hair may be achieved. A moisture level of about 30% is a balance between providing enough water to disrupt the hydrogen bonds to allow the hair to shape and not enough water that must be removed for the hydrogen bonds to be reformed.
Moisture sensing devices have been developed in the past to determine the moisture level in hair, and have relied on various techniques including resistance and capacitance measurements to obtain the desired indication. However, these methods only work well for a known cross sectional quantity and density of the hair being measured. As the hair density or compactness is varied, these measurement techniques fail. Additionally, these techniques rely primarily on the moisture content outside of the hair fiber for the measurement, and do not have the ability to accurately measure moisture content within hair fibers as well.
Thus, there is a need for a sensing device which can accurately and reliably determine the moisture content of a substrate, such as hair, including moisture both inside and outside of the hair fiber.